After sending many messages, I get many *** Email address is removed for privacy ***  Delivery status notification (faiilure) or something similar returns.  Opening this I get:

This is an automatically generated Delivery Status Notification.

Delivery to the following recipients failed.
 
       addressee emaill address.

and the body of the message that was sent.  No error codes or anything else!

This happens for about 25% of a mailing, and these addresses cannot be sent no matter what I do.  Sending through SMTP from hotmail account I have had for 20 years or so.

I can recapture these email addresses, but how can I send to them?